The present invention relates to a zoom lens. More particularly, the present invention relates to a zoom lens having a fixed total length that can most suitably be used as interchangeable lens and is advantageous for picking up moving images and also to a zoom lens that can be operated either in the first mode for allowing a wide focusing distance range up to infinity or in the second mode for allowing large imaging magnifications.
There has been a trend of providing digital cameras with a functional capability of picking up moving images in recent years and correspondingly there is a demand for interchangeable lenses that are adaptable to the camera bodies of digital cameras having a function of taking moving pictures.
Conventional still cameras are only required to operate satisfactorily at a good timing for taking a picture. In other words, once the person who is taking a picture determines framing and composition, his or her still camera is only required to put the
target for imaging in focus. Thus, the camera is equipped with functions that can achieve this objective. More specifically, still cameras are normally equipped with a phase difference autofocus (AF) function that makes the camera operate quickly and accurately for focusing.
However, as for picking up moving images, most video cameras for consumer use are required to be held in a focusing state by exploiting the AF function, although some professional video cameras may be excepted. A technique referred to as contrast AF method (so-called hill climbing method) using an image sensor is adopted to maintain a focusing state. With this technique, the focus lens is constantly moved forward and backward by a very small distance (an operation referred to as wobbling) from the in-focus position thereof to detect a change in the contrast and, when it is determined that there is a change in the focusing state, the focus lens is appropriately moved to put the focus lens back in focus once again. While the wobbling function constantly maintains the focus lens in focus if the distance between the lens and the target for imaging changes, the focus lens is required to be driven for wobbling at high speed depending on the frame rate of the camera body. In other words, the focus lens that operates for wobbling is required to be lightweight and designed to minimize the range of its wobbling motion. Additionally, such a wobbling operation accompanies a problem of noise generation and the generated noise is recorded as sound particularly when the camera is taking a moving picture. Therefore, how to realize a camera that operates for wobbling in silence is an important problem to be solved. Furthermore, if the image magnification changes remarkably while the focus lens is wobbling, the picked up image appears to be constantly swaying and hence very strange. Therefore, how to reduce the change in the magnification at the time of wobbling is another important problem to be solved.
Negative lead type zoom lenses in which a negative lens group that is advantageous for downsizing the camera is arranged closest to the target for imaging have been proposed. However, according to most of them, the first lens group having a negative refractive power is moved when the magnification is changed from the wide-angle end to the telephoto end. Generally, the first lens group has a large outer diameter and a large number of lenses so that it is heavy. While the heavy weight of the first lens group may not give rise to any problem in the case of manual zooming, a movable heavy first lens group is not desirable for electromotive zoom lenses that are employed for most video cameras. Therefore, it is requested that the first lens group is held unmovable at the time of zooming.
Zoom lenses in which the first lens group having a negative refractive power is held unmovable when changing the magnification (zooming) from the wide-angle end to the telephoto end are known and described in Japanese Patent No. 2,778,232, JP-A-11-167063 and JP-A-2010-211056. These zoom lenses are 5-group zoom lenses having negative, negative, positive, negative and positive lens groups arranged in the above mentioned order from the target for imaging side, in which the third lens group having a positive refractive power is moved toward the target for imaging side and, at the same time, the negative lens groups arranged in front and at the back of the third lens group and the fifth lens group having a positive refractive power are moved to adjust the position of the image surface.
Zoom lenses devised to operate in an imaging mode that enables infinity focusing and zooming and a macro imaging mode that enables imaging with large imaging magnifications are known. For instance, JP-A-2006-301474 discloses a zoom lens in which the lens groups other than the lens located closest to the target for imaging is moved toward the target for imaging in a macro mode to achieve large imaging magnifications.